ABSTRACT There are 2.5 million Americans who abuse or are dependent on opioids. Methadone maintenance treatment (MMT) is the most common treatment for opioid dependence. While MMT has been effective in improving drug use outcomes, the overwhelming majority continue to engage in unhealthy lifestyles (e.g., physical inactivity) that lead to significant mental and physical health morbidities such as cardiovascular disease, diabetes, hypertension, obesity, depression, sleep difficulties, and cognitive impairments. Given the mental and physical health benefits of physical activity (PA), interventions targeting increases in PA in MMT patients could have a significant impact on reducing the overall morbidity and mortality. The purpose of this study is to develop a feasible, acceptable and effective multilevel PA intervention that addresses both individual and interpersonal factors delivered in the context of a health care setting (i.e., methadone clinics). To do so, we are proposing to train MMT patients who are already engaging in PA at public health recommended levels to deliver a group-based PA intervention to physically inactive MMT patients at a large community-based methadone clinic. An increasing number of peer-facilitated PA interventions have been found to effectively increase and sustain physical activity levels, though none in substance abusing populations. Peers may play a particularly important role in increasing physical activity in MMT, as this population faces unique and significant barriers to PA (e.g., depression, smoking, triggers for drug use in environment). MMT peers who have successfully navigated through these barriers and are physically active can share information, help in problem solving barriers, act as role models, and offer support and encouragement, thereby helping inactive MMT patients increase self-efficacy and motivation for sustaining PA. The design of this study takes place in 2 phases -- the R21 phase will be focused on the development of a peer-facilitated PA+Fitbit intervention (Peer-PA+Fitbit), while the R33 phase will evaluate the efficacy of Peer- PA+Fitbit in an RCT. The R21 phase will consist of: 1) focus groups with potential peers and physically inactive MMT patients; 2) development of a PA manual that integrates use of the Fitbit; 3) developing a training protocol for peers; 4) identifying safe, walkable paths in participant neighborhoods, and 5) a 12-week open pilot trial (n=20) to test the feasibility and acceptability of the Peer-PA+Fitbit intervention. The R33 Phase will involve randomizing 150 patients currently receiving MMT to one of three arms: 1) Peer-PA+Fitbit intervention, 2) contact control wellness groups (Well+Fitbit), and 3) Usual Care (UC). Follow-up assessments will be conducted at 3, 6, & 12-months to determine both the short- and long-term adherence to physical activity. We expect that this project will lead to the development of a scalable, multilevel peer-facilitated PA intervention tailored to patients in MMT, thereby improving the overall health and well-being of this at-risk population.